1. Field of the Invention
One disclosed aspect of the embodiments relates to a switching power supply and, in particular, is useful for a switching power supply equipped with a burst mode for stopping switching operation for a certain period of time.
2. Description of the Related Art
There has been known a switching power supply which controls output voltage using an integrated circuit (IC) as a low voltage power supply for an electronic apparatus. In recent years, there has been trend of further reducing power in the standby period for operation of an electronic apparatus to demand to reduce the power consumption of the switching power supply itself. As a configuration for reducing the power consumption of the switching power supply, Japanese Patent Application Laid-Open No. 2008-245419 discusses a system for controlling switching operation (also referred to as burst operation) in such a manner that the voltage level across a feedback terminal is set to be varied depending on a load condition on the output side of the switching power supply to decrease an ON period and increase an OFF period of the switching power supply at the time of a light load.
Operation states of the switching power supply include three states: a heavy load state in which an application apparatus is in operation; a light load state in which the apparatus is not in operation and a part thereof is stopped to reduce power consumption; and a middle load state in which the apparatus is on standby so as to be able to start whenever it is required. It is effective to perform the burst operation in the light load state to further reduce power consumption in the middle load state. However, the switching power supply which performs the burst operation in the middle load state to reduce power consumption has the following problems to be solved.
Since a load current is relatively large in the middle load state of the switching power supply, a change is also large in an output voltage in an OFF period (a period for which the burst operation is forcibly turned off) of a switching element (a field effect transistor, for example) and in a voltage across the feedback terminal of an power supply IC controlling the operation of the switching element. For this reason, a pulse width of the switching element at the time of ON state also becomes long. A long pulse width increases an instantaneous electric power to be transferred to a secondary side via an insulation transformer and also increases change in an output voltage in an ON period of the switching element and a fed back voltage. As a result, the ripple voltage of the output voltage is increased.
In general, a trade off relationship exists between the reduction of power consumption in the switching power supply and an increase in the ripple voltage of the output voltage. The ripple voltage tends to be large in a recent switching power supply that attaches importance to the reduction of power consumption.